Since a rail joint portion (rail weld zone) is most easily damaged in a rail, the rail joint portion requires maintenance cost. Further, a rail joint portion is a main generation source of noise and vibration that are generated during the passage of a train. Since the speed and trainload of a train are increasing in various countries, a technique, which forms a long rail by continuously connecting rail joint portions having the above-mentioned problem by welding, is being used generally.
A general rail will be described with reference to FIGS. 1A and 1B. FIG. 1A is a side view of a long rail. A long rail is manufactured by welding at least two rails. For this reason, the long rail has a weld zone 7. A weld bead 8 exists at the weld zone 7.
FIG. 1B is a cross-sectional view taken along a line A-A shown in FIG. 1A. As shown in FIG. 1B, a rail includes a head portion 1 (rail upper portion) with which a wheel comes into contact, a foot portion 3 (rail lower portion) that is placed on sleepers, and a rail web portion 2 that is formed between the head portion 1 and the foot portion 3. Further, the head portion 1 includes a head-top portion 4, and the foot portion includes a foot-surface portion 5 and a sole portion 6.
Flash butt welding (for example, Patent Document 1), gas pressure welding (for example, Patent Document 2), enclosed arc welding (for example, Patent Document 3), and Thermit welding (for example, Patent Document 4) are main methods of welding a rail.
FIGS. 2A to 2C are views illustrating flash butt welding. As shown in FIGS. 2A to 2C, the flash butt welding generates arc between end faces by applying a voltage to materials 10, which are to be welded and are disposed so as to face each other, through electrodes 9, and melts the end faces of the materials to be welded. Further, when the materials to be welded are sufficiently heated, the materials are pressed against each other in an axial direction, so that the materials to be welded are joined to each other.
FIGS. 3A and 3B are views illustrating Thermit welding. FIG. 3B is a cross-sectional view taken along a line B-B shown in FIG. 3A. In the Thermit welding, as shown in FIGS. 3A and 3B, materials 10 to be welded are disposed so as to face each other with a gap of 20 to 30 mm interposed therebetween and the gap is surrounded by a mold 14. Further, molten steel 16, which is generated in a crucible 15 by the reaction between aluminum and an iron oxide, is poured into the mold, so that the end faces of the rail are melted and welded to each other.
FIGS. 4A to 4C are views illustrating gas pressure welding. In the gas pressure welding, as shown in FIG. 4A, while joint surfaces are pressed, portions of materials to be welded in the vicinity of the joint surfaces are heated from side surfaces by burners 17 and the joint surfaces come into press contact with each other at high temperature. As shown in FIG. 4B, portions in the vicinity of a weld zone are deformed so as to expand by pressing. Further, as shown in FIG. 4C, expanding portions are removed by trimmers 18.
FIGS. 5A to 5B are views illustrating enclosed arc welding. In the enclosed arc welding, as shown in FIGS. 5A and 5B, materials to be welded are disposed so as to face each other with a gap of 10 to 20 mm interposed therebetween and backing metal 19 and siding metal 20 are disposed around the gap. Further, weld metal is built at the gap with a welding rod 21. This method is a so-called manual arc welding method.
In particular, there is a concern that fatigue cracks are generated from the neutral axis of the rail weld zone as an origination at a railway where a heavy-load freight train passes, a railway of a cold area, or the like. Accordingly, rails need to be frequently replaced in order to prevent the brittle fracture of a rail that is caused by the fatigue cracks. An example of brittle fracture is shown in FIGS. 6A and 6B.
FIG. 6A is a view showing a fatigue crack 22 that is generated at the rail web portion in a horizontal direction, and a brittle crack 23 that is caused by the fatigue crack. Further, FIG. 6B is a view showing the crack surface of the brittle crack 23 and the fatigue crack 22 shown in FIG. 6A. The fatigue crack 22 is generated from a weld defect as an origination, which is formed in the vicinity of the weld bead 8 and the neutral axis, in the horizontal direction. After the brittle crack 23 caused by the fatigue crack 22 penetrates the rail web portion in a thickness direction, one end of the crack is growing toward a rail head-top portion and the other end thereof is growing toward the foot portion. The origination of the fatigue crack 22 is not limited to the weld defect, and various causes are considered as the origination of the fatigue crack.
It is considered that the generation of a fatigue crack is affected by not only an external load condition but also residual stress in a material. FIG. 7A is a graph showing the distribution of residual stress at a rail weld zone in a circumferential direction. FIG. 7A shows that tensile residual stress exists when residual stress is larger than 0, and compressive residual stress exists when residual stress is smaller than 0.
From FIG. 7A, it is understood that large tensile residual stress in the circumferential direction (that is, vertical direction) of the rail is generated in the vicinity of the rail web portion of the rail weld zone by welding. Accordingly, it is considered that a fatigue crack generated from a weld defect as an origination is generated since a load is repeatedly applied to the vicinity of the rail web portion of the rail weld zone having large tensile residual stress by the passage of a train. In order to prevent the fatigue crack, it is preferable to prevent the generation of a weld defect an origination and to make a weld defect be ineffective even though the weld defect exists.
Further, FIG. 7B shows a relationship between a distance from a welding center (in the longitudinal direction of the rail) and residual stress that exists at the rail web portion of the rail in the vertical direction. From FIG. 7B, it is understood that large tensile residual stress exists in the range between the welding center and a position distant from the welding center by a distance of about 25 mm.
A track of a railway includes rails and sleepers that support the rails. When a train passes on the rails, dispersed loads are applied to the rails from a plurality of wheels of the train.
A cause, which generates the above-mentioned fatigue cracks, is related with the state of a load that is applied to the rail weld zone from the wheel. A load which is applied to the rail during the passage of the train varies at a rail portion immediately above the sleeper 24 and a rail portion that is formed between two sleepers 24. A vertical load of the train is directly applied to the rail at the rail portion immediately above the sleeper 24. When a long rail welded at a factory is installed on sleepers in the field, the position of the weld zone may correspond to that of the sleeper by accident. It is considered that several points where the positions of the weld zones correspond to those of the sleepers exist on a long rail having a length of several hundred meters.
FIG. 9A illustrates a time point where a wheel 25 passes just above the sleeper 24 (on a weld zone) at a point where the position of the sleeper 24 corresponds to the position of the weld zone. In this case, the largest stress is generated at the rail web portion 2 of which the cross-sectional area is small. The stress in this case is compressive stress, but large tensile residual stress exists at the rail web portion 2 as described above. Accordingly, while the rail web portion 2 receives actual tensile stress, stress repeatedly acts on the rail web portion.
Meanwhile, FIG. 9B illustrates a time point where a wheel 25 passes between two sleepers 24 (on a weld zone) at a point where the positions of the sleepers 24 and 24 do not correspond to the position of the weld zone. In this case, a load, which presses and bends the rail, is applied to the rail from the wheel 25 from above. For this reason, compressive stress in the longitudinal direction is generated at the rail head portion 1 and tensile stress in the longitudinal direction is generated at the rail foot portion 3. Bending stress applied to the rail web portion 2 is in neutral. Since the tensile stress of the rail foot portion 3 is generated whenever the wheel 25 passes, it is necessary to consider the generation of a fatigue crack at the rail foot portion 3.
FIG. 8 shows residual stress that is generated at the peripheral portion of the weld zone in the longitudinal direction. As shown in FIG. 8, large compressive stress in the longitudinal direction remains on the bottom of the rail. For this reason, even though tensile stress is applied to the bottom of the rail when a train passes, the tensile stress and the compressive stress offset each other in the state of effective stress. Accordingly, it is possible to suppress the generation of fatigue cracks. For this reason, an actual example of fatigue failure from the rail foot portion is uncommon. However, if compressive residual stress is small, damage from the fatigue crack 26 which is generated at the rail sole as an origination may be generated as shown in FIGS. 10A and 10B.
Patent Document 5 and Patent Document 6 disclose a method of making the entire rail weld zone or the head portion and the rail web portion of the rail weld zone in a high-temperature state by welding heat or heat transferred from the outside, and then performing accelerated cooling in order to prevent the damage to a rail web portion. According to this technique, since residual stress of a rail weld zone is controlled, it is possible to reduce tensile residual stress that is generated at the rail web portion of the rail weld zone in the vertical direction or to convert the tensile residual stress into compressive residual stress. For this reason, it is possible to improve the fatigue strength of the rail weld zone. It is possible to suppress the generation of the fatigue cracks from the rail web portion by this technique.
As other techniques that improve the fatigue strength of the rail weld zone, there are a method using shot peening as described in, for example, Patent Document 7, methods using hammer peening, grinder treatment, and TIG dressing, and the like.
Further, Patent Document 8 discloses a device for cooling a rail weld zone.
In order to improve the durability of a long rail, it is necessary to suppress the generation of fatigue cracks from a rail web portion and a foot portion of a weld zone and to give fatigue resistance to the long rail at the same time.
When the accelerated cooling of a head portion and a rail web portion of a rail weld zone is performed by a cooling method disclosed in Patent Document 5 and Patent Document 6, the tensile residual stress of the rail web portion in the vertical direction is improved, so that the generation of fatigue cracks at the rail web portion is suppressed. However, in the drawings of the Non-patent Document 1, it is indicated that the residual stress of a sole portion in the longitudinal direction of a rail is converted into tensile residual stress, if the above-mentioned method is employed. In recent years, since heavy-load trains have tended to increase, the burden that is caused by a bending load and applied to a sole portion, is increasing. Since the sole portion is tensioned in the longitudinal direction of a rail by the burden that is caused by the bending load, the fatigue strength of the rail sole portion is important. As described above, residual stress of the rail in the longitudinal direction significantly affects the fatigue strength of the rail sole portion. However, since residual stress of the rail sole portion in the longitudinal direction of the rail is reduced (is to be converted into tensile residual stress) in the cooling treatment of Patent Document 5 and Patent Document 6 as described above, there is a concern that fatigue strength is reduced. For this reason, there is a concern that damage shown in FIGS. 10A and 10B is generated.
Meanwhile, according to shot-peening treatment that is the related art for improving residual stress (that is, for applying compressive residual stress) by mechanical rail web-treatment, steel spheres, which have a diameter of several mm, are bumped against a material to plastically deform a surface layer of the material, so that the surface layer is subjected to work hardening. As a result, it is possible to improve fatigue strength by increasing residual stress. However, this treatment requires large facilities that project steel spheres, collect the steel spheres, and prevent dust, and the like. For this reason, the application of the shot-peening treatment to a large weld zone is limited. In addition, since materials to be projected need to be supplied due to the abrasion and damage thereof, it is disadvantageous in terms of cost.
Further, according to hammer peening that plastically deforms a weld zone by hitting a material with ends of tools, compressive stress is given to the material and stress concentration is suppressed by plastic deformation, so that fatigue strength of the material is improved. However, vibration is large during hitting, burden on a worker is large, and it is difficult to perform fine control and uniform treatment. In Non-patent Document 2, it is indicated that an effect of improving the fatigue strength is small due to a wrinkly groove portions, which are formed by working according to the treatment conditions.
Further, since grinder treatment suppresses stress concentration by smoothing weld bead toes, a reliable effect can be expected. However, if the weld bead toes are excessively ground, the thickness of a weld zone is insufficient, which causes reduction in strength. For this reason, there is a drawback in that grinder treatment requires skill and a long time.
Further, weld bead toes are melted again by arc generated from a tungsten electrode and are solidified again in a smooth shape in TIG dressing, so that stress concentration is suppressed. As a result, it is possible to improve fatigue strength. However, when a high-carbon material such as a rail is manually welded, a hard and brittle martensite structure is apt to be generated. In order to prevent the generation of a martensite structure, strict working management is required.
Further, it is possible to increase the hardness of a weld zone by performing appropriate cooling from a high-temperature state after welding by a device for cooling a rail weld zone disclosed in Patent Document 8. Meanwhile, according to the examination of the inventors, in order to control the state of residual stress of a weld zone, it is necessary to perform cooling in an appropriate range at appropriate intensity. It is considered that residual stress is also changed by the device of Patent Document 8, but cooling conditions for appropriate distribution of residual stress are not described.
Since a rail joint portion (rail weld zone) is most easily damaged in a rail as described above, the rail joint portion requires maintenance cost. Further, a rail joint portion is a main generation source of noise and vibration that are generated during the passage of a train. Since the speed and trainload of a train are increasing in various countries, a technique, which forms a long rail by continuously connecting rail joint portions having the above-mentioned problem by welding, is being used generally.
Flash butt welding (for example, see Patent Document 1), gas pressure welding (for example, see Patent Document 2), enclosed arc welding (for example, see Patent Document 3), and Thermit welding (for example, see Patent Document 4) are main methods of welding a rail.
When a rail joint portion is welded, in particular, there is a concern that fatigue cracks are generated in the vicinity of the neutral axis of the rail weld zone at a railway where a heavy-load freight train passes, a railway of a cold area, or the like. Accordingly, rails need to be frequently replaced in order to prevent the brittle fracture of a rail that is caused by the fatigue cracks. An example of brittle fracture is shown in FIGS. 41A and 41B. FIG. 41A shows a state where a fatigue crack 151 generated in a horizontal direction is generated in the vicinity of the neutral axis of a rail weld zone 150. A brittle crack 152 is generated toward the rail head portion and the rail foot portion. FIG. 41B shows a fracture surface of the fatigue crack 151 and the brittle crack 152. From FIG. 41B, it is understood that a fatigue crack 151 is generated from the vicinity of the neutral axis of the rail weld zone 150 as an origination and the brittle crack 152 then penetrates the rail web portion in a thickness direction. Meanwhile, in this specification, a rail upper portion 160 coming into contact with a wheel is referred to as a “head portion”, a rail lower portion 162 coming into contact with a sleeper is referred to as a “foot portion”, and a portion 161 formed between the head portion and the foot portion is referred to as a “rail web portion” (see FIGS. 27A and 27B).
It is considered that the generation of a fatigue crack is affected by not only an external load condition but also residual stress in a material. FIG. 42 is a graph showing the distribution of residual stress, which is caused by flash butt welding, at a peripheral portion of a rail weld zone in a circumferential direction. In a graph of FIG. 42, a positive direction of a vertical axis represents tensile residual stress and a negative direction of the vertical axis represents compressive residual stress. From FIG. 42, it is understood that tensile residual stress of the rail web portion is large. If a rail weld zone is positioned on the sleeper, compressive stress in the vertical direction acts on the rail web portion during the passage of a train. However, large tensile stress in the vertical direction (rail cross-section circumferential direction) remains at the rail web portion. Accordingly, while the rail web portion receives actual tensile stress, stress repeatedly acts on the rail web portion. For this reason, fatigue cracks are apt to be generated at the rail web portion.
Patent Document 5 and Patent Document 6 disclose a method of making the entire rail weld zone or the head portion and the rail web portion of the rail weld zone in a high-temperature state by welding heat or heat transferred from the outside, and then performing accelerated cooling in order to prevent the damage to a rail web portion. According to this technique, since residual stress of a rail weld zone is controlled, it is possible to reduce tensile residual stress that is generated at the rail web portion of the rail weld zone in the vertical direction or to convert the tensile residual stress into compressive residual stress. For this reason, it is possible to improve the fatigue strength of the rail weld zone.
Further, as techniques that improve the fatigue strength of the rail weld zone, there is a method using shot-peening treatment as described in, for example, Patent Document 7. In the shot-peening treatment, steel spheres, which have a diameter of several mm, are projected to a material to plastically deform a surface layer of the material, so that the surface layer is subjected to work hardening. Accordingly, it is possible to improve fatigue strength by converting residual stress into compressive stress.
Further, Patent Document 8 discloses a device for cooling a rail weld zone. The device includes an air chamber that cools a head-top surface of a rail weld zone, an air chamber that cools head-side surfaces of the rail weld zone, and air chambers that cool an abdomen portion (rail web portion) and a bottom portion (foot portion) of the rail weld zone. Each of the air chambers is provided with a plurality of nozzles that ejects compressed air, and a nozzle for detecting temperature is provided in the middle of a nozzle group of the air chamber that cools the head-top portion.
The rail head portion suffers from wear due to the contact between a wheel and itself. In particular, wear is facilitated on a curved track by the relative slip that occurs between a wheel and a rail. For this reason, a heat-treated rail of which a rail head portion is hardened is frequently employed for a curved section. In the welding of the heat-treated rail, it is preferable that the same hardness as the hardness of a base material to be welded be obtained by performing the accelerated cooling of the rail head portion after welding in a temperature range until the completion of pearlite transformation from an austenite temperature region. When the accelerated cooling of the rail head portion is performed after welding, the accelerated cooling of the head portion and the rail web portion of the rail weld zone is performed, so that the residual stress of the rail web portion in the vertical direction is reduced (that is, compressive residual stress is increased). Accordingly, the generation of a fatigue crack of the rail web portion is suppressed. This method is disclosed in Non-patent Document 1. However, from the experiment of the inventions, it is found that residual stress of the rail web portion is not significantly reduced even though the accelerated cooling of the head portion and the rail web portion of the rail weld zone is performed.
Further, shot-peening treatment requires large facilities that project steel spheres, collect the steel spheres, and prevent dust, and the like. For this reason, the application of the shot-peening treatment to a large weld zone is limited. In addition, since the steel spheres are abraded and damaged, the steel spheres need to be supplied at regular intervals. Accordingly, there is a problem in that running cost is required.
Furthermore, from tests performed by the inventors, it is found that the residual stress of a rail web portion is not reduced and fatigue life is not much lengthened when the accelerated cooling of a rail weld zone is performed by the cooling device disclosed in Patent Document 8. That is, it is apparent that the residual stress of the rail weld zone cannot be reduced (compressive residual stress cannot be increased) unless cooling is performed in an appropriate range of the rail weld zone at an appropriate cooling rate.